Use very similar data to Kawashima et al. (1984); do not use some records because missing due to
recording and digitizing processes. Use equation and method (although do not check all 36 combinations
of forms of equation) used by Kawashima et al. (1984), see section 2.46.

X(M,Δ,GCi) is in gal. Coefficients are: c = -1.190 and for ground group 1 a = 117.0 and b = 0.268
and for ground group 2 a = 88.19 and b = 0.297 and for group ground 3 a = 13.49 and b = 0.402 with
σ = 0.253.